1. Field of the Invention
This invention relates to a control system for a power train including an engine and a transmission, and, more particularly, to a control system for a power train including an engine, which is associated with a mechanical throttle valve and an electric throttle valve, and an automatic transmission.
2. Description of Related Art
Conventionally, automobile engines are provided with throttle valves which are opened in accordance with depressed strokes of an accelerator pedal so as to control the amount of intake air, i.e. the output torque of the engine. Such a throttle valve is mechanically coupled to an accelerator pedal by means of a wire or rod so as to be opened and closed in accordance with depressed strokes of the accelerator pedal. These types of throttle valves are conventionally well known in the art and are referred to as mechanical throttle valves.
Ordinarily, with such a mechanical throttle valve, the opening of the throttle valve is proportional to the depressed stroke of the accelerator pedal. However, with regard to intake air, the amount of intake air generally changes at relatively great rates for small depressed strokes of the accelerator pedal and tends to changes at small rates for large depressed strokes of the accelerator pedal. When depressing the accelerator pedal beyond a predetermined specific stroke, the amount of intake air does not increase. Hence, if utilization is made of only a mechanical throttle valve, it is actually very difficult to provide an engine output directly proportional to a depressed stroke of the accelerator pedal.
If the characteristic of engine output relative to the stroke of the accelerator pedal can be properly established to correspond to vehicle driving conditions, then a measurable improvement can be accomplished in the performance of driving force. However, with the mechanical throttle valve, once the change in opening of the mechanical throttle valve relative to the stroke of an accelerator pedal is established, it remains fixed and can not be varied. Consequently, it is difficult to variably obtain an appropriate engine output relative to vehicle driving conditions.
From this background, there has been proposed an electromagnetic actuator for opening and closing the throttle valve. With this type of throttle valve, an opening of the throttle valve is controlled in accordance with a control signal corresponding to a depressed stroke of the accelerator pedal so as to provide an amount of intake air, and hence engine output torque, directly proportional to the opening. By the aid of this type of electric throttle valve, the opening of the throttle valve, and hence the engine output, relative to the stroke of the accelerator pedal is variably established by modifying of an electric signal. Such an electric throttle valve is known from, for instance, Japan Unexamined Patent Publication No. 2-221658.
Where utilization of this type of electric throttle valve is adopted, then improved reliability can be obtained through the use of a combination of a mechanical throttle valve and an electric throttle valve. In such a set-up, because of a high reliability, the mechanical throttle valve serves as a primary or main throttle valve, and the electric throttle valve serves as a secondary or supplemental throttle valve. For instance, the mechanical throttle valve and the electric throttle valve are aligned in series in an intake passage of the engine. Such an arrangement is known from, for instance, Japan Unexamined Patent Publication No. 2-221658.
By means of controlling of the amount of air taken these two throttle valves arranged in the intake passage, an excellent engine response can be obtained corresponding to the stroke of the accelerator pedal. Moreover, the engine output can be controlled much more precisely by the combination of these valves than by the conventional throttle valve. In addition, the combination of these valves enables the precise coordination of changes in engine output so as to respond to vehicle driving conditions.
When a power train is constructed with an engine endowed with these two throttle valves and an automatic transmission, precise control exerted over the engine output can be utilized so as to realize a substantially improved feeling in shifting of an automatic transmission. That is, when the automatic transmission upshifts chronologically, for instance, from a first gear to a fourth gear, if the accelerator pedal remains stationary, a drop in engine output torque ordinarily occurs following each upshift. In other words, there is an unavoidable occurrence of torque shock accompanying each upshift of the automatic transmission. Such shift shock adversely affects to feelings of shifting of the automatic transmission.
In an attempt to eliminate this problem, in the power train described above, an electronic throttle valve has a valve opening characteristic different for various gears of the automatic transmission. The throttle valve is restrictively regulated in opening to be slightly smallish prior to shifting up of the automatic transmission so as to restrict the driving force of the power train to some extent in a lower gear, for instance the first gear, prior to the upshifting. On the other hand, after shifting up of the automatic transmission, the electric throttle valve is regulated in opening to be largish so as to increase the driving force of the power train in a higher gear, for instance the second gear, following the upshifting. In such a way, torque shock is mitigated during upshifting with a resultant improvement in shift feelings. It is general, even for the power trains which includes engines provided with two throttle valves disposed in intake lines, to provide gear shift control schedules on the basis of opening of the mechanical throttle valve and speed of the drive system (i.e. vehicle speed) in order to defer as much as possible to the intentions or desires of vehicle drivers.
However, in a power train in which torque control during shifting is accomplished by appropriately controlling the opening of an electric throttle valve, if the electric throttle valve is operationally impeded due, for instance, to trouble of an actuator, then, the torque control during shifting can not be appropriately accomplished, which constitutes a source of instability in shift control. Especially, in a power train in which the opening of the electric throttle valve is set to be smaller or less than the opening of the mechanical throttle valve for engine output torque restriction, if the electric throttle is impeded or causes an operational failure at large openings, the output torque of the engine cannot be effectively restricted, making shift control unstable.
In addition, automatic transmissions of the type having a so called "overdrive mode," in which engine speed is increased and transmitted to wheels, can not transmit fully the engine output torque to the wheel owing to the torque transmissive capacity or clutch capacity of transmission mechanisms. For this reason, a power train including such an automatic transmission is controlled to restrict the engine output torque so as not to exceed the torque transmissive capacity of transmission mechanism.
When the power train is comprised of this type of automatic transmission and an engine which has two throttle valves disposed in an intake line, ordinarily, by restrictively controlling the electric throttle valve in opening to be smallish, cutting or restriction of the engine output torque is exercised in the overdrive mode. However, with this type of power train, if the electric throttle valve is operationally impeded at large openings, the torque cutting control can not be effectively exercised over the overdrive mode.